Cut the brioche loaf into slices that are 1 inch thick. Each slice should feel sturdy enough to lift with one hand without bending at the center.
Whisk the eggs in a medium bowl, then blend in the milk, maple syrup, brown sugar, or granulated sugar, vanilla, and cinnamon before pouring the custard into a shallow dish. The mixture should look creamy and uniform, with the cinnamon dispersed throughout.
Lay one brioche slice in the custard for 5 to 10 seconds, turn it over, and soak the second side for 5 to 10 seconds. Lift it when both surfaces are coated and the slice still holds its shape.
Melt 1 Tablespoon of butter in a large skillet over medium-low heat, then cook the dipped slices in uncrowded batches for 3 to 4 minutes on the first side and 2 to 3 minutes after flipping. The toast is ready to turn when it slides freely under a spatula, and the finished center should feel gently puffed beneath a golden crust.
Move cooked slices to a baking sheet in a 200°F oven while finishing the remaining batches. Keep the slices spaced apart so their bottoms do not soften from trapped steam.
Serve the French toast immediately with any desired toppings. Maple syrup, confectioners’ sugar, berries, whipped cream, blueberry sauce topping, or strawberry sauce topping all work well with the warm cinnamon custard.